There are already known, from the prior art, automatically gripping closures referred to as having hooks in hooks. In particular when they are used to close a bag of plastics material, these closures have a number of disadvantages. On the one hand, they lack much flexibility, the edges of the opening facing each other and carrying the respective hooked strips having, when the devices of the prior art are used, a large degree of rigidity, which is disadvantageous to the use of the plastics bag. Furthermore, these closures having hooks in hooks of the prior art have the additional disadvantage, in the case of plastics bags which are intended to contain small products, in particular in powdered, liquid or granulated form, that small particles, such as grains or dust, have a tendency to become trapped under the hooks of the hooked strips. Consequently, these small particles which are jammed below the hooks may impair the correct operation of the closure. If retained for a long time below the hooks, they can further bring about contamination of the product contained in the bag when it is reused for a product of another type. Furthermore, in the prior art, the only existing plastics bags which comprise such hook-in-hook closures comprise hooked strips which have been produced by means of a moulding method which is complicated to implement, in particular owing to difficulties during removal from the mould.